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Encyclopedia > Renal physiology
This illustration demonstrates the normal kidney physiology. It also includes illustrations showing where some types of diuretics act, and what they do.

Renal physiology is the study of the physiology of the kidneys. Image File history File links Size of this preview: 800 Ã— 505 pixelsFull resolution (950 Ã— 600 pixel, file size: 40 KB, MIME type: image/gif) I made this image myself. ... Image File history File links Size of this preview: 800 Ã— 505 pixelsFull resolution (950 Ã— 600 pixel, file size: 40 KB, MIME type: image/gif) I made this image myself. ... This article or section does not cite any references or sources. ... The kidneys are the organs that filter wastes (such as urea) from the blood and excrete them, along with water, as urine. ...

## Functions of the kidney GA_googleFillSlot("encyclopedia_square");

The functions of the kidney can be divided into two groups: secretion of hormones, and extracellular homeostasis.

### Secretion of hormones

Erythropoietin (IPA pronunciation: , alternative pronunciations: ) or EPO is a glycoprotein hormone that is a cytokine for erythrocyte (red blood cell) precursors in the bone marrow. ... â€œRed cellâ€ redirects here. ... This article does not cite any references or sources. ... Not to be confused with rennin, the active enzyme in rennet. ... The renin-angiotensin system (RAS) or the renin-angiotensin-aldosterone system is a hormone system that helps regulate long-term blood pressure and blood volume in the body. ... Vitamin D is a fat soluble vitamin that contributes to the maintenance of normal levels of calcium and phosphorus in the bloodstream. ... Vitamin D is a fat soluble vitamin that contributes to the maintenance of normal levels of calcium and phosphorus in the bloodstream. ... E1 - Alprostadil I2 - Prostacyclin A prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. ...

### Extracellular homeostasis

The kidney is responsible for maintaining a balance of several substances:

 Substance Description Proximal tubule Loop of Henle Distal tubule Collecting duct glucose If glucose is not reabsorbed by the kidney, it appears in the urine, in a condition known as glucosuria. This is associated with diabetes mellitus.[1]. reabsorption (almost 100%) via sodium-glucose transport proteins[2] (apical) and GLUT (basolateral). - - - amino acids Almost completely conserved.[3] reabsorption - - - urea Regulation of osmolality. Varies with ADH[4][5] reabsorption (50%) via passive transport secretion - reabsorption in medullary ducts sodium Uses Na-H antiport, Na-glucose symport, sodium ion channels (minor)[6] reabsorption (65%, isosmotic) reabsorption (25%, thick ascending, Na-K-2Cl symporter) reabsorption (5%, sodium-chloride symporter) reabsorption (5%, principal cells), stimulated by aldosterone chloride Usually follows sodium. Active (transcellular) and passive (paracellular)[7] reabsorption reabsorption (thin ascending, thick ascending, Na-K-2Cl symporter) reabsorption (sodium-chloride symporter) - water Uses aquaporin. See also diuretic. - reabsorption (descending) - reabsorption (with ADH, via arginine vasopressin receptor 2) bicarbonate Helps maintain acid-base balance. [8] reabsorption (80-90%) [9] reabsorption (thick ascending) [10] - reabsorption (intercalated cells, via band 3 and pendrin) protons Uses vacuolar H+ATPase - - - secretion (intercalated cells) potassium Varies upon dietary needs. reabsorption (65%) reabsorption (20%, thick ascending, Na-K-2Cl symporter) - secretion (common, via Na+/K+-ATPase, increased by aldosterone), or reabsorption (rare, hydrogen potassium ATPase) calcium Uses calcium ATPase, sodium-calcium exchanger reabsorption reabsorption (thick ascending) via passive transport - - magnesium Calcium and magnesium compete, and an excess of one can lead to excretion of the other. reabsorption reabsorption (thick ascending) reabsorption - phosphate Excreted as titratable acid. reabsorption (85%) via sodium/phosphate cotransporter[11]. Inhibited by parathyroid hormone. - - -

The body is very sensitive to its pH level. Outside the range of pH that is compatible with life, proteins are denatured and digested, enzymes lose their ability to function, and the body is unable to sustain itself. The kidneys maintain acid-base homeostasis by regulating the pH of the blood plasma. Gains and losses of acid and base must be balanced. Acids are divided into "volatile acids"[12] and "nonvolatile acids".[13] See also titratable acid. In the biology of the kidney, the proximal convoluted tubule is the segment of the renal tubule that drains Bowmans capsule. ... In the kidney, the loop of Henle is the portion of the nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. ... Kidney nephron The distal convoluted tubule (DCT) is a portion of kidney nephron between the loop of Henle and the collecting duct system. ... The collecting duct system of the kidney consists of: The connecting tubule The cortical collecting duct The medullary collecting duct Categories: Urinary system ... Glucose (Glc), a monosaccharide (or simple sugar), is an important carbohydrate in biology. ... Glycosuria or glucosuria is a condition of osmotic diuresis typical in those suffering from diabetes mellitus. ... For the disease characterized by excretion of large amounts of very dilute urine, see diabetes insipidus. ... Sodium-glucose transport proteins are a family of glucose transporter found in the intestinal mucosa of the small intestine (SGLT1) and the proximal tubule of the nephron (SGLT2). ... Alpha intercalated cell The apical membrane of a polarized cell is the part of the plasma membrane that forms its lumenal surface, distinct from the basolateral membrane. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... The basolateral membrane of an epithelial cell is the part of the plasma membrane that forms its basal and lateral (not apical) surfaces. ... In chemistry, an amino acid is any molecule that contains both amino and carboxylic acid functional groups. ... Urea is an organic compound with the chemical formula (NH2)2CO. Urea is also known as carbamide, especially in the recommended International Nonproprietary Names (rINN) in use in Europe. ... In chemistry, the osmole (Osm) is a non-SI unit of measurement that defines the number of moles of a chemical compound that contribute to a solutions osmotic pressure. ... Passive transport bobo means of moving biochemicals, and other atomic or molecular substances, across membranes. ... For sodium in the diet, see Edible salt. ... The sodium-hydrogen antiporter is a protein found in the nephron of the kidney. ... Sodium channels (also known as voltage-gated sodium channels) are integral membrane proteins that are localized in and conduct sodium ions (Na+) through a cells plasma membrane. ... In chemistry, the osmole (Osm) is a non-SI unit of measurement that defines the number of moles of a chemical compound that contribute to a solutions osmotic pressure. ... The Na-K-2Cl symporter is an ion pump carrier protein that is inhibited by loop diuretics. ... The sodium-chloride symporter is a symporter ion pump used primarily to remove sodium and chloride ions from the distal convoluted tubule of the kidney. ... Aldosterone is a steroid hormone (mineralocorticoid family) produced by the outer-section (zona glomerulosa) of the adrenal cortex in the adrenal gland to regulate sodium and potassium balance in the blood. ... The chloride ion is formed when the element chlorine picks up one electron to form an anion (negatively-charged ion) Clâˆ’. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. ... For sodium in the diet, see Edible salt. ... Paracellular transport refers to the transfer of substances between cells of an epithelium. ... The Na-K-2Cl symporter is an ion pump carrier protein that is inhibited by loop diuretics. ... The sodium-chloride symporter is a symporter ion pump used primarily to remove sodium and chloride ions from the distal convoluted tubule of the kidney. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Sideview of Aquaporin 1 (AQP1) Channel Aquaporins are a class of integral membrane proteins or more commonly referred to as a class of major intrinsic proteins (MIP) that form pores in the membrane of biological cells. ... This illustration shows where some types of diuretics act, and what they do. ... Arginine vasopressin receptor 2 (AVPR2) is a protein that acts as receptor for arginine vasopressin. ... For baking soda, see Sodium bicarbonate In inorganic chemistry, a bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. ... Anion Exchanger 1 (AE1) or Band 3 is a phylogenetically preserved transport protein responsible for catalysing the electroneutral exchange of chloride (Cl-) for bicarbonate (HCO3-) across a plasma membrane. ... Pendrin is an ion exchanger found in the cortical collecting duct. ... For alternative meanings see proton (disambiguation). ... V-ATPase schematic Vacuolar type H+-ATPase (V-ATPase) is a highly conserved evolutionarily ancient enzyme with remarkably diverse functions in eukaryotic organisms. ... General Name, symbol, number potassium, K, 19 Chemical series alkali metals Group, period, block 1, 4, s Appearance silvery white Standard atomic weight 39. ... The Na-K-2Cl symporter is an ion pump carrier protein that is inhibited by loop diuretics. ... Simplified Diagram of the sodium pump Na+/K+-ATPase (also known as the Na+/K+ pump or Na+/K+ exchanger) is an enzyme (EC 3. ... Aldosterone is a steroid hormone (mineralocorticoid family) produced by the outer-section (zona glomerulosa) of the adrenal cortex in the adrenal gland to regulate sodium and potassium balance in the blood. ... Gastric hydrogen potassium ATPase also know as H+/K+ ATPase // Function and location The gastric hygrogen potassium ATPase or H+/K+ ATPase is the proton pump of the stomach and as such is the enzyme primarily responsible for the acidification of the stomach contents. ... For other uses, see Calcium (disambiguation). ... Calcium ATPase is a form of ATPase which transfers calcium after a muscle has contracted. ... The sodium-calcium exchanger (often denoted Na+/Ca2+ exchanger or exchange protein) is an antiporter ion pump membrane protein which removes calcium from cells. ... Passive transport bobo means of moving biochemicals, and other atomic or molecular substances, across membranes. ... General Name, symbol, number magnesium, Mg, 12 Chemical series alkaline earth metals Group, period, block 2, 3, s Appearance silvery white solid at room temp Standard atomic weight 24. ... A phosphate, in inorganic chemistry, is a salt of phosphoric acid. ... Titratable acid is a term to describe acids such as phosphoric acid, sulfuric acid which are involved in renal physiology. ... The sodium/phosphate cotransporter (or Na+-Pi cotransport) is a protein found in the proximal tubule of the nephron. ... Parathyroid hormone (PTH) is secreted by the parathyroid glands as a polypeptide containing 84 amino acids. ... For other uses, see PH (disambiguation). ... The body is very sensitive to its pH level. ... Blood plasma is the liquid component of blood, in which the blood cells are suspended. ... Titratable acid is a term to describe acids such as phosphoric acid, sulfuric acid which are involved in renal physiology. ...

The major homeostatic control point for maintaining this stable balance is renal excretion. The kidney is directed to excrete or retain sodium via the action of aldosterone, antidiuretic hormone (ADH, or vasopressin), atrial natriuretic peptide (ANP), and other hormones. Abnormal ranges of the fractional excretion of sodium can imply acute tubular necrosis or glomerular dysfunction. Homeostasis is the property of either an open system or a closed system, especially a living organism, which regulates its internal environment so as to maintain a stable, constant condition. ... Aldosterone is a steroid hormone (mineralocorticoid family) produced by the outer-section (zona glomerulosa) of the adrenal cortex in the adrenal gland to regulate sodium and potassium balance in the blood. ... Antidiuretic hormone (ADH), or arginine vasopressin (AVP), is a peptide hormone produced by the hypothalamus, and stored in the posterior part of the pituitary gland. ... Atrial natriuretic factor (ANF), atrial natriuretic peptide (ANP) or atriopeptin, is a polypeptide hormone involved in the homeostatic control of body water and sodium. ... The fractional excretion of sodium (FENa) is a measure of the percentage of sodium excreted in the urine versus the sodium reabsorbed by the kidney. ... Acute tubular necrosis may be toxic or ischemic. ... The glomerulus is a capillary bed found surrounded by the Bowmans capsule of the nephron in the vertebrate kidney. ...

## Mechanisms

The kidney's ability to perform many of its functions depends on the three fundamental functions of filtration, reabsorption, and secretion.

### Filtration

The blood is filtered by nephrons, the functional units of the kidney. Each nephron begins in a renal corpuscle, which is composed of a glomerulus enclosed in a Bowman's capsule. Cells, proteins, and other large molecules are filtered out of the glomerulus by a process of ultrafiltration, leaving an ultrafiltrate that resembles plasma (except that the ultrafiltrate has negligible plasma proteins) to enter Bowman's space. Filtration is driven by Starling forces. For other uses, see Blood (disambiguation). ... A nephron is the basic structural and functional unit of the kidney. ... A renal corpuscle is the initial filtering component of a nephron in the kidney. ... Glomerulus refers to two unrelated structures in the body, both named for their globular form. ... The Bowmans capsule(other names: capsula glomeruli, glomerular capsule) is a cup like sac at the beginning of the tubular component of a nephron in the mammalian kidney. ... In biological terms, Ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowmans capsule in the kidneys. ... Blood proteins are proteins found in blood plasma. ... Formulated in 1896 by the British physiologist Ernest Starling, the Starling equation illustrates the role of hydrostatic and oncotic forces (the so-called Starling forces) in the movement of fluid across capillary membranes. ...

The ultrafiltrate is passed through, in turn, the proximal tubule, the loop of Henle, the distal convoluted tubule, and a series of collecting ducts to form urine. In the biology of the kidney, the proximal convoluted tubule is the segment of the renal tubule that drains Bowmans capsule. ... In the kidney, the loop of Henle is the portion of the nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. ... The distal convoluted tubule (DCT) is a portion of kidney nephron between the loop of Henle and the collecting duct system. ... The collecting duct system of the kidney consists of: The connecting tubule The cortical collecting duct The medullary collecting duct Categories: Urinary system ... This article is about the urine of animals generally. ...

#### Renal plasma threshold

The renal plasma threshold is the minimum plasma concentration of a substance that results in the excretion of that substance in the urine. Blood plasma is the liquid component of blood, in which the blood cells are suspended. ...

For example, the renal plasma threshold for glucose is 180 to 200 mg per 100 ml. Glycosuria (sugar in urine) results when the plasma glucose concentration reaches and exceeds the renal plasma threshold of glucose. When the plasma glucose concentration is very high, the filtered glucose can saturate the carriers and reach the transport maximum of that molecule. Any amount past the transport maximum will continue through the renal tubules and be excreted in the urine.

### Reabsorption

Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. It is called reabsorption (and not absorption) because these substances have already been absorbed once (particularly in the intestines). In anatomy, the intestine is the segment of the alimentary canal extending from the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine. ...

Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium (the connective tissue that surrounds the nephrons), and then the transport of these substances from the interstitium into the bloodstream. These transport processes are driven by Starling forces, diffusion, and active transport. Sodium-Potassium pump, an example of Primary active transport secondary active transport Active transport (sometimes called active uptake) is the mediated transport of biochemicals, and other atomic/molecular substances, across membranes. ... Passive transport bobo means of moving biochemicals, and other atomic or molecular substances, across membranes. ... Formulated in 1896 by the British physiologist Ernest Starling, the Starling equation illustrates the role of hydrostatic and oncotic forces (the so-called Starling forces) in the movement of fluid across capillary membranes. ... diffusion (disambiguation). ... Sodium-Potassium pump, an example of Primary active transport secondary active transport Active transport (sometimes called active uptake) is the mediated transport of biochemicals, and other atomic/molecular substances, across membranes. ...

#### Indirect reabsorption

In some cases, reabsorption is indirect. For example, bicarbonate (HCO3-) does not have a transporter, so its reabsorption involves a series of reactions in the tubule lumen and tubular epithelium. It begins with the active secretion of a hydrogen ion (H+) into the tubule fluid via a Na/H exchanger: The sodium-hydrogen antiporter is a protein found in the nephron of the kidney. ...

• In the lumen
• The H+ combines with HCO3- to form carbonic acid (H2CO3)
• Luminal carbonic anhydrase enzymatically converts H2CO3 into H2O and CO2
• CO2 freely diffuses into the cell
• In the epithelial cell
• Cytoplasmic carbonic anhydrase converts the CO2 and H2O (which is abundant in the cell) into H2CO3
• H2CO3 readily dissociates into H+ and HCO3-
• HCO3- is facilitated out of the cell's basolateral membrane

Carbonic anhydrase (carbonate dehydratase) is a family of metalloenzymes (enzymes that contain one or more metal atoms as a functional component of the enzyme) that catalyze the rapid interconversion of carbon dioxide and water into carbonic acid, protons, and bicarbonate ions. ... Facilitated diffusion (facilitated transport) is a process of passive transport (diffusion) via which molecules diffuse across membranes, with the help of transport proteins (mediated transport). ... is a membrane This page is a candidate for speedy deletion. ...

#### Hormones

Some key regulatory hormones for reabsorption include:

Both hormones exert their effects principally on the collecting ducts. Aldosterone is a steroid hormone (mineralocorticoid family) produced by the outer-section (zona glomerulosa) of the adrenal cortex in the adrenal gland to regulate sodium and potassium balance in the blood. ... Antidiuretic hormone (ADH), or arginine vasopressin (AVP), is a peptide hormone produced by the hypothalamus, and stored in the posterior part of the pituitary gland. ... The collecting duct system of the kidney consists of: The connecting tubule The cortical collecting duct The medullary collecting duct Categories: Urinary system ...

### Secretion

Tubular secretion is the transfer of materials from peritubular capillaries to renal tubular lumen. Tubular secretion is caused mainly by active transport. In the renal system, peritubular capillaries are tiny blood vessels that travel along side nephrons allowing reabsorption and secretion between blood and the inner lumen of the nephron. ... Sodium-Potassium pump, an example of Primary active transport secondary active transport Active transport (sometimes called active uptake) is the mediated transport of biochemicals, and other atomic/molecular substances, across membranes. ...

Usually only a few substances are secreted. These substances are present in great excess, or are natural poisons.

## Measurement of renal function

Main article: Renal function

A simple means of estimating renal function is to measure pH, blood urea nitrogen, creatinine, and basic electrolytes (including sodium, potassium, chloride, and bicarbonate). As the kidney is the most important organ in controlling these values, any derangement in these values could suggest renal impairment. In medicine (nephrology) renal function is an indication of the state of the kidney and its role in physiology. ... For other uses, see PH (disambiguation). ... The blood urea nitrogen (BUN) test is a measure of the amount of nitrogen in the blood that comes from urea. ... Creatinine is a breakdown product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass). ... An electrolyte is a substance containing free ions that behaves as an electrically conductive medium. ... For sodium in the diet, see Edible salt. ... General Name, symbol, number potassium, K, 19 Chemical series alkali metals Group, period, block 1, 4, s Appearance silvery white Standard atomic weight 39. ... The chloride ion is formed when the element chlorine picks up one electron to form an anion (negatively-charged ion) Clâˆ’. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. ... For baking soda, see Sodium bicarbonate In inorganic chemistry, a bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. ...

There are several more formal tests and ratios involved in estimating renal function:

 Measurement Calculation Details renal plasma flow $RPF = frac{effective RPF}{extraction ratio}$ [14] Volume of blood plasma delivered to the kidney per unit time. Para-aminohippuric acid (PAH) is a renal analysis tool used to provide an estimate. renal blood flow $RBF = frac{RPF}{1 - HCT}$ (HCT is hematocrit) Volume of blood delivered to the kidney per unit time. In humans, the kidneys together receive roughly 20% of cardiac output, amounting to 1 L/min in a 70-kg adult male. glomerular filtration rate GFR = Kf([Pc − Pi] − σ[πc − πi]) (estimation using Starling equation) Volume of fluid filtered from the renal glomerular capillaries into the Bowman's capsule per unit time. Estimated using inulin. Usually a creatinine clearance test is performed but other markers, such as the plant polysaccharide inulin or radiolabelled EDTA, may be used as well. filtration fraction $FF = frac{GFR}{RPF}$ [15] Measures efficiency of reabsorption. anion gap AG = [Na+] - ([Cl-] + [HCO3-]) Cations minus anions. Excludes K+ (usually), Ca2+, H2PO4-. Aids in the differential diagnosis of metabolic acidosis Clearance (other than water) $C = frac{UV}{P}$ where U = concentration, V =urine volume / time, U*V = urinary excretion, and P = plasma concentration [16] Rate of removal free water clearance C = V − Cosm or $V - frac{U_{osm}}{P_{osm}}V$ $C_{H_2O}$[17][18] The volume of blood plasma that is cleared of solute-free water per unit time. Net acid excretion $NEA = V ( U_{NH_4} + U_{TA} - U_{HCO_3} )$ Net amount of acid excreted in the urine per unit time

## References

1. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch06/7ch06p11.htm
2. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch05/7ch05p13.htm
3. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch06/7ch06p17.htm
4. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch06/7ch06p10.htm
5. ^ http://www2.kumc.edu/ki/physiology/course/five/5_1.htm
6. ^ http://www2.kumc.edu/ki/physiology/course/six/6_1.htm
7. ^ http://www2.kumc.edu/ki/physiology/course/six/6_1.htm
8. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch06/7ch06p08.htm
9. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch12/7ch12p21.htm
10. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch12/7ch12p22.htm
11. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch05/7ch05p13.htm
12. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch12/7ch12p12.htm
13. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch12/7ch12p13.htm
14. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch04/7ch04p28.htm
15. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch04/7ch04p17.htm
16. ^ http://www2.kumc.edu/ki/physiology/course/four/4_2.htm
17. ^ http://sprojects.mmi.mcgill.ca/nephrology/resources/freewater.asp
18. ^ http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch08/7ch08p21.htm

Results from FactBites:

 © The American Physiological Society - AJP Renal Physiology (372 words) The American Journal of Physiology - Renal Physiology publishes original manuscripts on a broad range of subjects relating to the kidney, urinary tract, and their respective cells and vasculature, as well as to the control of body fluid volume and composition. Thomas R. Kleyman, MD, is a Professor of Medicine, Cell Biology and Physiology, and Pharmacology, and Chief of the Renal-Electrolyte Division at the University of Pittsburgh. His postdoctoral training in transport physiology and membrane biochemistry was at Columbia University.
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